PROJECT SUMMARY/ABSTRACT Malaria remains one of the most devastating infectious diseases of the world, underscoring the need to develop effective vaccines. The current candidate malaria vaccines against the liver stages induce CD8+ T- cell-mediated protection. However, what remains unknown is the manner in which the anti-malarial CD8+ T cells are elicited in vivo. This unanswered question is particularly prominent in view of a very recent study showing that only through intravenous administration (and no other routes) do radiation-attenuated sporozoites (IrSpz) induce a potent malaria-specific CD8+ T-cell response in the livers of monkeys and of mice and provide anti-malarial protection in mice. Therefore, it appears that the nature of vaccine vectors, as well as the routes of vaccination, influences the mode of induction of protective anti-malarial CD8+ T cells in vivo. The overall aim of this proposal is to determine the mechanisms of in vivo induction of anti-malarial CD8+ T cells. SYVPSAEQI, derived from the P. yoelii circumsporozoite (PyCS) protein, is to date the only known CD8+ epitope that mediates protection against P. yoelii infection in mice and is presented by an H-2Kd molecule. Therefore, in addressing our overall goal, we have generated C57BL/6 transgenic (Tg) mice, in which Kd molecule is expressed only on dendritic cell (DC) (CD11c-Kd), macrophage (huCD68-Kd), or hepatocyte (Alb- Kd), by using CD11c promoter, huCD68 promoter, or albumin promoter, respectively. We have also generated MHC-I-Kd Tg mice that express a Kd molecule under the MHC-I promoter, in which we could induce a potent, protective anti-malarial immunity, dependent on both the PyCS protein and CD8+ T cells. These MHC-I-Kd Tg mice will be used as a positive control. In the proposed study, we will immunize the Kd Tg mice with malaria vaccines, including an adenovirus expressing the PyCS antigen, IrPySpz, or live PySpz followed by treatment with chloroquine, by different routes. We will determine the quantity, quality, and durability of PyCS antigen- specific CD8+ T-cell response induced in each group of Kd Tg mice in Aim 1. In Aim 2, we will challenge these immunized Kd Tg mice with live malaria parasites to determine the level and persistence of protective immunity induced in vivo. In Aim 3, we will determine which Kd-expressing cells induce the protective anti-malarial immunity by isolating these Kd+ cells from immunized, various Kd Tg mice, and adoptively transferring them to nave MHC-I-Kd Tg mice, followed by a malaria challenge. Finally, we will isolate PyCS antigen-specific CD8+ T cells from immunized, various Kd Tg mice and adoptively transfer them to nave MHC-I-Kd Tg mice, followed by a malaria challenge, to determine the protective capacity of the CD8+ T cells in Aim 4. Overall, we believe that the identification of the induction mechanisms of anti-malarial protective CD8+ T cells could ultimately lead to the vastly improved designs of potent T-cell-based vaccines against human malaria.